A notice of proposed rulemaking (NPRM) to amend part 39 of the Federal Aviation Regulations (14 CFR part 39) by superseding AD 94-15-12, amendment 39-8983 (59 FR 37933, July 26, 1994), and AD 94-15-18, amendment 39-8989 (59 FR 41233, August 11, 1994), which are applicable to certain Boeing Model 747 series airplanes, was published in the Federal Register on March 12, 2003 (68 FR 11764). The NPRM proposed to continue to require that the FAA- approved maintenance inspection program be revised to include inspections that will give no less than the required damage tolerance rating (DTR) for each structural significant item, and repair of cracked structure. The NPRM also proposed to require additional and expanded inspections, and repair of cracked structure. Additionally, the NPRM also proposed to expand the applicability of the existing ADs to include additional airplanes. \n\nDefinitions \n\n\tFor the purposes of the discussions following in the "Comments" section of this AD, references to Boeing Document No. D6-35022, "Supplemental Structural Inspection Document," (SSID) for Model 747 Airplanes, Revision G, dated December 2000, are referred to as "Revision G." \n\nComments \n\n\tInterested persons have been afforded an opportunity to participate in the making of this amendment. Due consideration has been given to the comments received. \n\nRequests To Allow Training Flights Equivalent \n\n\tTwo commenters request that two training flights be considered equivalent to one revenue flight for all Structural Significant Items (SSIs), except SSIs F-46, F-49, F-50, F-51, W-3, S-1, S-2, and E-1 through E-10. One of the commenters, the manufacturer, states that analyses show that for all SSIs, except for the above excluded SSIs, fatigue damage accumulated during a touch-and-go training flight conducted at less than 2.0 pounds per square inch (psi) internal cabin pressure is significantly less than half of the fatigue damage accumulated on a typical revenue flight. \n\n\tThe FAA does not concur with the commenters' request. In this case, we do not consider it appropriate to include various provisions in an AD applicable to a unique use of an affected airplane. We have determined that for clarity of the final rule, such a request is best evaluated through submitting a request for alternative methods of compliance as provided for in paragraph (h)(1) of this AD. \n\nRequest To Extend the Repetitive Intervals \n\n\tOne commenter, an operator, notes that paragraph (c) of the NPRM does not allow the provisions to increase task repetitive intervals by 10%, as specified in paragraph 5.1.8 of Revision G. The commenter requests that such provisions be allowed to accommodate unanticipated scheduling requirements similar to the provisions allowed in the Corrosion Prevention and Control Program (CPCP) required by AD 90-25- 05, amendment 39-6790, (55 FR 49268, November 27, 1990). \n\n\tWe do not agree that the repetitive inspection interval may be increased up to 10% without further evaluation. Any unsubstantiated increases in the task repetitive intervals may not maintain the level of safety this AD requires. The task repetitive intervals in Revision G are based on the assumption that the entire Boeing Model 747 fleet is inspected at a minimum with the required DTR prescribed in the document. Therefore, any unsubstantiated increases in the task repetitive intervals will lower the corresponding DTR to below the minimum required, which may invalidate the methodology employed in the inspection program. However, we do agree that, on a case-by-case basis, the repetitive inspection interval, which may include interim instructions, may be extended to accommodate unanticipated scheduling requirements. We will consider requests for adjustment of the compliance time that maintains an acceptable level of safety per paragraph (h)(1) of this AD. \n\nRequests To Revise the Cost Impact \n\n\tOne commenter requests that a more extensive cost breakdown be provided. The commenter states that the cost of complete repetition of the whole SSID program for every D-Check is not included in the cost estimates of the NPRM. The commenter concludes, therefore, that it will require more than three times the number of work hours specified in the NPRM to perform the SSID program completely. A second commenter states that, based on its experience, it takes approximately 3,500 work hours per airplane to accomplish the initial inspection of all SSIs during a D-Check and 6,600 work hours per airplane during a C-Check. The second commenter also points out that it would require additional ground time to accomplish the inspections to ensure the availability of non- destructive testing (NDT) inspectors and because of the maintenance limitations during the x-ray inspections. The second commenter also notes that further costs would be incurred because the additional ground time would reduce airplane utilization. \n\n\tWe acknowledge that the cost estimate of work hours specified in the NPRM may be too low. Based on the commenters' information and experience and the fact that approximately 25% of the airplanes will be able to accomplish the initial inspection during a D-Check, we agree to increase the estimated work hours to accomplish the inspections from 1,275 to 5,825 work hours. We point out, however, that the compliance time specified in this AD should allow ample time for the inspections to be accomplished at the same time as scheduled inspections and maintenance for the majority of affected operators, which will minimize the costs associated with special airplane scheduling. We provide the cost estimate of a single inspection cycle because there is no way to accurately project how many repetitive inspections would be necessary for all affected airplanes. Clearly, based on the "life" of each affected airplane, the number of required repetitive inspections would vary. \n\n\tWe recognize that this AD will take many work hours toaccomplish, and we acknowledge that maintaining airplanes in an airworthy condition is vital, but sometimes expensive. ADs require specific actions to address specific unsafe conditions and consequently may appear to impose costs that would not otherwise be borne by operators. However, because operators have a general obligation to maintain their airplanes in an airworthy condition, this appearance is deceptive. Attributing those costs solely to this AD is unrealistic because, in the interest of maintaining safe airplanes, prudent operators would accomplish these actions even if they were not required by the AD. We cannot provide a further break-down of costs, since the commenter did not provide such information, and we have not received any additional cost information from any other source. \n\nRequest To Revise Paragraph (e) of the NPRM \n\n\tOne commenter, the manufacturer, requests that we revise paragraph (e) of the NPRM to provide authorization for Boeing Designated Engineering Representatives (DERs) to approve repair methods. The commenter suggests the following rewrite: \n\n\t"(e) Damage found during any inspection required by this AD shall be repaired prior to further flight per a method approved by the Manager, Seattle Aircraft Certification Office (ACO), FAA; or per data meeting the type certification basis of the airplane approved by a Boeing Company DER who has been authorized by the Manager, Seattle ACO, to make such findings. For a repair method to be approved, the approval must specifically reference this AD." \n\n\tWe acknowledge that authorization to approve repairs may be delegated to certain Boeing DERs. However, we do not agree to replace the wording in paragraph (e ) of this AD that specifies repairing the structure per an FAA-approved method. Repairs approved by Boeing DERs with an FAA Form 8110-3 are, by definition, "FAA-approved." This AD also allows use of other FAA-approved repairs, including repairs described in the Boeing Structural RepairManual and repairs approved by other qualified DERs. Therefore, no change is necessary to the AD to allow approval by an authorized Boeing Company Designated Engineering Representative. \n\nRequest To Clarify Requirements of Section 6.0 of Revision G \n\n\tOne commenter requests that the NPRM be clarified to state that Section 6.0, "SSI Discrepancy Reporting" is also a requirement. The commenter also requests that we include the section number in paragraph (c) of the NPRM that is being referred to, because paragraph 5.3 of Revision G does not refer to "Damage Tolerant Rating (DTR) System Application." Additionally, the commenter requests that the sections be stated in sequential order as they appear in Revision G. The commenter believes that Section 6.0 should be clearly stated in the requirements, since many of the affected airplanes are not of U.S. registry and would not be required to provide mechanical reliability reports under CFR part 121.703. \n\n\tWe agree that clarification is necessary and have revised paragraph (c) of the AD to specify that revision of the maintenance or inspection program shall include and shall be implemented per the procedures in Section 5.0, "Damage Tolerance Rating (DTR) System Application," and Section 6.0, "SSI Discrepancy Reporting" of Revision G, excluding paragraphs 5.1.2; 5.1.6, item 5; 5.1.8; 5.2; 5.2.1; 5.2.2; 5.2.3; and 5.2.4. \n\n\tHowever, since the "DTR System Application" is the subject of all of Section 5.0, we do not consider it to be an issue with labeling and sequencing of the paragraphs of Section 5.0 of Revision G. No change to the AD is necessary in this regard. \n\nRequest To Clarify Paragraph (c) of the NPRM \n\n\tOne commenter requests clarification on whether phased inspections are permitted under the requirements of paragraph (c) of the NPRM. The commenter acknowledges that the NPRM does exclude paragraph 5.2 of Revision G; however, paragraph 5.1.11 is included in the NPRM and that paragraph refers back to paragraph 5.2 of Revision G. The commenter notes that paragraph 5.1.11 states, in part, "* * * inspections shall be accomplished at frequency F but not necessarily on 100 percent of the operator's affected fleet." The commenter states that it believes that the goal is to move away from a sample-based approach to a threshold-based approach to be consistent with other Boeing airplane models. \n\n\tWe agree that it is necessary to clarify that phased inspections are not permitted. We have added a new Note 4 to the AD clarifying that, even though paragraph 5.2 of Revision G is referenced in paragraph 5.1.11, paragraph 5.2 is still excluded as a method of compliance with the requirements of this AD. \n\nRequests To Revise the Initial Inspection Compliance Time \n\n\tSeveral commenters suggest using alternative compliance time schedules. Two commenters state that the compliance time specified in the NPRM does not reflect the existing candidate fleet program for damage tolerance based inspections that has been in place for 19 years. One commenter believes that the proposed actions specified in the NPRM are an exploratory effort to detect unknown cracking. Further, the commenter states that the thresholds and intervals specified in the Supplemental Structural Inspection Program (SSIP) are purely analytical and do not reflect the fact that the candidate fleet inspection program has been providing real data feedback. Another commenter expressed agreement with these comments. Several commenters believe that the compliance time for the transition from the current candidate fleet program to the threshold based program specified in the NPRM can be phased in over a longer period of time. One of the commenters considers the compliance times in the NPRM to be too stringent. Another commenter suggests that since it has accomplished the SSID inspections on 22 airplanes and has found only known defects, the compliance time can be extended longer than 1,000 flight cycles. Yet another commenterstates that the grace period would impose significant costs and scheduling difficulties on operators because many of the specified inspections are scheduled similar to D-Check inspections. \n\n\tWe do not agree with the commenters' requests to extend the compliance times. The SSIP is based on a certain probability that cracking will be found on the inspected fleet before the cracking initiates in other airplanes that have not been inspected. High-cycle airplanes in the fleet are more likely to experience initial fatigue damage. The current candidate fleet approach has resulted in a statistically invalid number of airplanes being inspected; therefore, \nwe do not concur that an extended phase-in period for initial inspection of high-cycle airplanes provides an acceptable level of safety. As mentioned in the preamble of the NPRM, the threshold required by the existing AD for the candidate fleet is much lower, 12,000 total flight cycles for Model 747SR and 10,000 total flight cycles for Model 747-100 and -200 series airplanes, than that specified in this AD. Additionally, the commenters do not provide any statistical information on how the participation level of the current SSID candidate program provides an acceptable level of safety. Therefore, no change to the final rule is necessary regarding the specified compliance times. \n\nRequests To Inspect a Sample of the Fleet \n\n\tSeveral commenters request that a percentage of the fleet, as specified in the DTR form, be inspected at a maximum interval specified by the D-Check maintenance schedule. The commenters state that paragraph 5.1.11 of Revision G establishes a D-Check maximum frequency be applied to a percentage of an operator's fleet, depending upon the DTR. Removing the percent sampling while maintaining the D-Check maximum frequency, results in unnecessarily forcing repeat inspections at shorter intervals than that indicated by the DTR form. \n\n\tWe do not agree. For reasons discussed in the NPRM and earlier inthis preamble, we have considered the candidate fleet approach and have moved to a threshold approach. In doing so, we require inspections of all SSIs when the threshold has been reached. Only inspecting a sample of SSIs where the damage tolerance rating (DTR) provides an interval greater than a D-Check would not provide an acceptable level of safety. If operators wish to request an adjustment to the compliance time, they may do so according to the provisions specified in paragraph (h)(1) of this AD. Such requests should include a new proposed inspection interval and must include data to substantiate that such an adjustment would provide an acceptable level of safety. No change is necessary to the final rule in this regard. \n\nRequests To Remove the D-Check Cap \n\n\tSeveral commenters request that we remove the proposed requirement to perform all applicable SSID tasks on every airplane at the maximum interval of a D-Check or equivalent time, as specified in paragraph 5.1.11 of Revision G. One commenter states that such a requirement creates an undue burden for the operator because more inspections would have to be performed than if performed under the technical requirements of the SSID program where sampling is permitted. Another commenter asserts that such a requirement does not conform to other analytical methods to define a necessary inspection interval. The commenter asserts that the D-Check capping requirement would lead to a significant burden for operators that have a shorter interval at the fourth, fifth, and subsequent D-Checks. One commenter poses the following condition as an example: The 5th D-Check is equivalent to approximately 2,500 flight cycles. The SSID estimates that a D-Check is approximately 6,000 flight cycles. Therefore, it is the commenter's understanding that the inspection interval could be increased for some SSID items to higher intervals than the intervals of the D-Check, without decreasing the level of safety below the required DTR. Thecommenter also states that, by increasing the inspection interval and removing the sampling concept at the same time, the entire SSID program will be easier to incorporate, understand, and track. Further, the commenter asserts that cost reduction can be achieved by omitting certain inspections that are not necessary at each D-Check. Another operator states that the proposed requirement will require operators to repeat some inspections unnecessarily. \n\n\tWe do not agree with removing the D-Check cap from the AD. The D- Check cap will provide confidence in the existing analytical methods by providing more than one inspection on SSIs with long repetitive intervals. One-time inspections at a threshold do not give the confidence that cracking will not develop on aging airplanes that have accumulated flight cycles beyond the design service objective (DSO). However, for operators that have shorter intervals for their later D- Checks, we will consider requests for alternative methods of compliance (AMOCs) in accordance with paragraph (h)(1) of this AD. \n\nRequest To Redefine SSI \n\n\tOne commenter, the manufacturer, requests that the definition of SSI as specified in the NPRM be redefined from "principal structural element," to a "principal structural element as listed in Revision G of the SSID D6-35022." \n\n\tWe do not agree. Revision G defines an SSI as a principal structural element (PSE). Further, Revision G of the SSID does not say that an SSI is a "principal structural element as listed in Revision G." No change to the final rule is necessary in this regard. \n\nRequest To Redefine PSE \n\n\tOne commenter, an operator, requests that the definition of a PSE in Note 3 of the NPRM be revised to read: "Any detail, element, or assembly, which contributes significantly to the carrying of flight, ground, pressurization or control loads and whose failure could affect the structural integrity necessary for the safety of the aircraft." The commenter points out that there aremany published definitions of PSE, and that confusion may occur as a result. The commenter requests that we provide one consistent definition and considers that the definition used in the Maintenance Steering Group 3 (MSG 3), Revision 2b, to be the industry standard definition. The commenter also notes that Boeing Model 747 series airplanes have recently been subject to a MSG 3, Revision 2b program review. \n\n\tWe do not agree with the commenter's request. We consider that the definition provided in Advisory Circular 25.571-1C, dated April 29, 1998, to be the standard, and that is the definition provided in this AD. \n\nRequest To Revise Compliance Times of Parts Replaced With New Structures \n\n\tOne commenter, an operator, requests that we add paragraph 5.1.17 of Revision G to the paragraphs that are excluded from the requirements of paragraph (c) of the NPRM, or that Boeing change paragraph 5.1.17 of Revision G to specify 20,000 flight cycles or 10,000 flight cycles from part replacement, whichever is later. The commenter notes that paragraph 5.1.17 of Revision G refers to the inspection requirements for the portion of an SSI that has been replaced with new structure, and that the inspection may be deferred until a new threshold of 10,000 flight cycles are accumulated. The commenter states that, in some cases, the replaced structure would have to be inspected prior to the threshold specified in the NPRM. The commenter points out that the 10,000 flight cycle threshold is consistent with the requirements of AD 94-15-12, since the inspections are required to begin upon the accumulation of 10,000 total flight cycles for airplanes in the candidate fleet. \n\n\tWe acknowledge the commenter's position and recognize that clarification is necessary. It is not our intent to have operators inspect replaced structure prior to the threshold of the AD. To clarify that intent, we have revised paragraph (d) of the AD by adding paragraph (d)(3) to the AD to specify that, for the portion of an SSI that has been replaced with new structure, the inspections can be deferred until the later of the times specified in paragraph (d)(3)(i) or (d)(3)(ii) of the AD, as applicable. We have added this clarification to paragraph (d) of the AD, since it also includes compliance times for wing structure and all other structures. Additionally, clarifying paragraph (d) of the AD will prevent time lost in issuance of the AD due to a delay in having Boeing revise and republish Revision G. \n\nConclusion \n\n\tAfter careful review of the available data, including the comments noted above, the FAA has determined that air safety and the public interest require the adoption of the rule with the changes previously described. The FAA has determined that these changes will neither increase the economic burden on any operator nor increase the scope of the AD. \n\nChanges to 14 CFR Part 39/Effect on the AD \n\n\tOn July 10, 2002, the FAA issued a new version of 14 CFR part 39 (67 FR 47997, July 22, 2002), which governs the FAA's airworthiness directives system. The regulation now includes material that relates to altered products, special flight permits, and alternative methods of compliance (AMOCs). Because we have now included this material in part 39, only the office authorized to approve AMOCs is identified in each individual AD. However, for clarity and consistency in this final rule, we have retained the language of the NPRM regarding that material. \n\nChange to Labor Rate Estimate \n\n\tWe have reviewed the figures we have used over the past several years to calculate AD costs to operators. To account for various inflationary costs in the airline industry, we find it necessary to increase the labor rate used in these calculations from $60 per work hour to $65 per work hour. The cost impact information, below, reflects this increase in the specified hourly labor rate. \n\nCost Impact \n\n\tThere are approximately 1,000 airplanes of the affected design in the worldwidefleet. \n\n\tThe FAA estimates that 87 airplanes of U.S. registry are currently affected by the actions that are currently required by AD 94-15-12 and AD 94-15-18. We estimate that it takes approximately 1,000 work hours per airplane to accomplish, at an average labor rate of $65 per work hour to accomplish those actions. Based on these figures, the cost impact of the currently required actions on U.S. operators is estimated to be $5,655,000, or $65,000 per airplane, per inspection cycle. \n\n\tWe estimate that 181 airplanes of U.S. registry are affected by this AD. The new actions that are required by this new AD will take approximately 5,825 work hours per airplane to accomplish, at an average labor rate of $65 per work hour. Based on these figures, the cost impact of the new requirements of this AD on U.S. operators is estimated to be $68,531,125, or $378,625 per airplane, per inspection cycle. \n\n\tThe cost impact figures discussed above are based on assumptions that no operator hasyet accomplished any of the requirements of this AD action, and that no operator would accomplish those actions in the future if this AD were not adopted. \n\n\tThe number of work hours, as indicated above, is presented as if the accomplishment of the actions in this AD are to be conducted as "stand alone" actions. However, in actual practice, these actions for the most part will be accomplished coincidentally or in combination with normally scheduled airplane inspections and other maintenance program tasks. Therefore, the actual number of necessary additional work hours will be minimal in many instances. Additionally, any costs associated with special airplane scheduling will be minimal. \n\nRegulatory Impact \n\n\tThe regulations adopted herein will not have a substantial direct effect on the States, on the relationship between the national Government and the States, or on the distribution of power and responsibilities among the various levels of government. Therefore, it is determined that this final rule does not have federalism implications under Executive Order 13132. \n\n\tFor the reasons discussed above, I certify that this action (1) is not a "significant regulatory action" under Executive Order 12866; (2) is not a "significant rule" under DOT Regulatory Policies and Procedures (44 FR 11034, February 26, 1979); and (3) will not have a significant economic impact, positive or negative, on a substantial number of small entities under the criteria of the Regulatory Flexibility Act. A final evaluation has been prepared for this action and it is contained in the Rules Docket. A copy of it may be obtained from the Rules Docket at the location provided under the caption ADDRESSES. \n\nList of Subjects in 14 CFR Part 39 \n\n\tAir transportation, Aircraft, Aviation safety, Incorporation by reference, Safety. \n\nAdoption of the Amendment \n\nAccordingly, pursuant to the authority delegated to me by the Administrator, the Federal Aviation Administration amends part 39 of the Federal Aviation Regulations (14 CFR part 39) as follows: \n\nPART 39--AIRWORTHINESS DIRECTIVES \n\n1. The authority citation for part 39 continues to read as follows: \n\n\tAuthority: 49 U.S.C. 106(g), 40113, 44701. \n\n§ 39.13 (Amended) \n\n2. Section 39.13 is amended by removing amendments 39-8983 (59 FR 37933, July 26, 1994) and 39-8989 (59 FR 41233, August 11, 1994), and by adding a new airworthiness directive (AD), amendment 39-13566, to read as follows: